Tobe & Raven, Embryology of Onagraceae (Myrtales) 
671 
tetrahedral. Decussate or isobilateral arrangements also occur, but at low frequencies. 
Pollen grains are two-celled when shed. 
Ovular orientation and integuments (Table 3): Tire ovule is anatropous and bitegmic, 
and the micropyle is formed by both integuments. For Stenosiphon, Johansen (1930b) 
reported that 'the inner integument is prolonged into a beak-like process,' and presented 
drawings of ovules showing the micropyle formed by the inner integument alone (1930b: 
319, Figs 6, 7 [sterile ovule]). However, we have confirmed that the micropyle of 
Steiiosiplion is also formed by both integuments. The micropyle appears to be formed 
largely by the endostome of a highly prolonged inner integument, but the outer 
integument is also prolonged and its bp exceeds that of the inner integument. A more 
or less prolonged inner (and outer) integument of tlris kind was observed in other 
genera of tribe Onagreae (e.g., Calylophus) and is not restricted to Stcnosiphon as a 
characterisbc feature. At any rate, its appearance accounts for Johansen's misinterpretabon. 
Early in ontogeny both the inner and the outer integument arise almost 
simultaneously, or the inner integument arises a little earlier than the outer one, 
from an ovular primordium (see Tobe & Raven 1985: 452, Figs lA, B; 459, Fig. 4A). 
In subsequent stages of development in most genera (generally up to the megaspore 
mother cell stage), the two integuments grow together, so that the tip of the inner 
integument exceeds that of the outer integument or reaches the top of nucellus 
earlier than the latter (Fig. 3). However, in the species examined of Epilobium 
(Figs 4, 5) and Gayophytum (Fig. 6) the development of the inner integument is 
extremely retarded. For instance, at the megaspore mother cell stage, the inner 
integument is much shorter than the outer one, whereas the tip of the outer 
integument reaches near the top of the nucellus. In a few ovules of Oenothera flavn, 
the inner integument was somewhat shorter than the outer one, but most samples of 
Figs 1 and 2. Transverse sections of young anthers showing two different types of wall formation. 
1. Basic type (photo from Oenothera fniticosa). 2. Monocotyledonous type (photo from Clnrkia 
dudleyana). See text for explanation. Abbreviations: ct, endothecium; ep, epidermis; me, 
microspore mother cell; ml, middle layer; t, tapetum. Scales equal 10 pm. 
